The present invention relates to a pump for ultra-high pressure liquid chromatography, and more particularly to a pump to generate the high pressure and low flow rates required for ultra-high pressure liquid chromatography.
Pumps used in typical high pressure liquid chromatography (HPLC) generate approximately 5000 psi. These pumps, which typically use mechanical mechanisms to actuate flow-generating pistons, cannot create the pressures required for ultra-high pressure liquid chromatography (UHPLC), which operates at pressures in excess of 25,000 psi. Pneumatic amplifiers have been used to generate high pressures for UHPLC systems. For example, in the paper entitled xe2x80x9cUltrahigh-Pressure Reversed-Phase Liquid Chromatography in Packed Capillary Columns,xe2x80x9d Analytical Chemistry, 1997,69(6), 983-989, two Haskel pneumatic amplifier pumps are used to perform isocratic LC separations at pressures of 60,000 psi. FIG. 1 shows such a configuration. However, since the gas in pneumatic amplifiers is compressible, pneumatic amplifiers can deliver only constant pressure flow, which precludes high pressure gradient mixing necessary to perform gradient chromatography. That is, in gradient chromatography, two solvents are mixed in varying ratios, thereby requiring constant volume flow of the respective solvents, which cannot be done with pneumatic amplifier pumps. Alternatively, gradient chromatography can be performed by feeding the gradient mixture into a single pump. However, this is difficult to implement at the low flow rates used in UHPLC.
The paper entitled xe2x80x9cUltrahigh-Pressure Reversed-Phase Capillary Liquid Chromatography: Isocratic and Gradient Elution Using Columns Packed with 1.0-xcexcm Particles,xe2x80x9d Analytical Chemistry, 1999,71(3), 700-708, describes a UHPLC system that uses a mechanical actuator rather than a pneumatic actuator. The mechanical actuator comprises an ultrahigh-pressure constant-flow syringe pump including an electric motor connected to a gear reduction system and linear actuator that moves a piston. Although the syringe pump can generate operating pressures in excess of 130,000 psi, the mechanical actuator is too large to commercialize effectively.
The present invention provides a pump that can generate sufficient pressure and that creates substantially constant flow rate in a range sufficient for ultra-high pressure liquid chromatography.
The pump according to the invention comprises a hydraulic amplifier system using hydraulic force to generate high pressures in excess of approximately 30,000 psi and low flow rates of approximately 1 xcexcL/min or less. An HPLC pump is used to pump hydraulic fluid into a primary cylinder of the hydraulic amplifier. The primary cylinder contains a large-diameter piston, which actuates a small-diameter piston contained in a secondary cylinder. Due to the reduction in cross-sectional area of the pistons, the pressure in the primary cylinder is amplified. The amount of pressure amplification is determined by a ratio of the surface areas of the pistons. The reduction in cross-sectional area of the hydraulic amplifier also creates a reduction in flow rate, thereby achieving the desired low flow rate.
Features and advantages of the invention include provision of a pump configuration capable of generating a pressure in excess of approximately 30,000 psi and a flow rate of approximately 1 xcexcL/min or less. The pump configuration according to the present invention is suitable for use in an ultra-high pressure liquid chromatography system and is reliable and easy to manufacture. Standard of-the shelf HPLC pumping technology can be used to control the flow of the primary pumping fluid.
The configuration according to the invention may be implemented to avoid a pressure transducer in a secondary piston. Dead volumes may be advantageously minimized.
The present invention also can be used to perform a gradient separation at constant pressure, for example, when a solvent""s compressibility makes accurate flow control difficult.